Preparation and Electrochemical Properties of LiNi_0.89Co_0.09Al_0.02O₂ Submicron-Sized Single-Crystal and Superflexible High-Strength Buckypaper Cathode Composites for Lithium-Ion Battery

Single-crystalline ultra-fine sized LiNi_xCo_yAl_1-x-yO₂ (NCA) features stable Al─O bonds and crack-resistantance due to no grain boundaries, exhibiting excellent long charge–discharge cycle stability as a lithium-ion battery cathode. With flexible electronics growing, flexible energy storage materials are urgently needed. However, preparing single-crystalline ultra-fine NCA particles and assembling them into a self-supported superflexible cathode is still a tough challenge. Here, submicron-sized Ni-rich LiNi_0.89Co_0.09Al_0.02O₂ single-crystals were made via a molten-salt-assisted high-temperature solid-phase method with lithium added during annealing. Then, they were combined with carbon nanotubes by directional pressure filtration to form a superflexible porous hybrid buckypaper (BP). Under O₂, sintering at 750°C for 15 h with 1.03 Li addition and annealing with a Li source at 750°C for 5 h gave the optimal submicron-sized NCA single crystals. The BP could curl around a 2 mm-radius rod, bend 180°, knot without damage, and had 9.59 MPa tensile strength. In a lithium-ion battery, at 1C, its initial and 200-cycle capacities were respectively 192.27 and 179.3 mAh/g (93.2% retention); at 5C, respectively 135.4 and 132.9 mAh/g (98% retention). This work offers a new idea for self-supported flexible cathode construction.